Video encoding method and decoding method, apparatuses therefor, programs therefor, and storage media for recording the programs

ABSTRACT

A video encoding method, in which a video signal consisting of two or more signal elements is targeted to be encoded, includes a step of setting a downsampling ratio is set for a specific signal element in a frame, in accordance with the characteristics in the frame; and a step of generating a target video signal to be encoded, by subjecting the specific signal element in the frame to downsampling in accordance with the set downsampling ratio. The frame may be divided into partial areas in accordance with localized characteristics in the frame; and a downsampling ratio for a specific signal element in these partial areas may be set in accordance with the characteristics in each partial area.

TECHNICAL FIELD

The present invention relates to a video encoding method in which avideo signal consisting of two or more signal elements is targeted to beencoded and to an apparatus for the same, and also to a video decodingmethod for decoding encoded data generated in accordance with this videoencoding method and to an apparatus for the same, and also to a videoencoding program which is used to realize the video encoding method andto a computer-readable recording medium on which this program isrecorded, and also to a video decoding program which is used to realizethe video decoding method and to a computer-readable recording medium onwhich this program is recorded.

Priority is claimed on Japanese Patent Application No. 2006-276074,filed Oct. 10, 2006, the contents of which are incorporated herein byreference.

BACKGROUND ART

In standard encoding schemes for moving pictures starting withH.264/AVC, chrominance elements in input video which have beendownsampled when compared to the luminance elements (namely, which havebeen sampled at a lower frequency than the frequency at which theluminance elements were sampled) are used. This corresponds to videoformats known as 4:2:2, 4:2:0, and 4:1:1 formats.

Video formats in which these chrominance elements are resampled arebased on the physiological finding that visual sensitivity is lesssensitive towards chrominance elements than towards luminance elements,and have been introduced with the aims of reducing the amount of videoinformation and lowering the cost of imaging devices (see, for example,Non-patent documents 1 and 2).

On the other hand, because of increased expectations for furtherimprovements in video image quality, 4:4:4 format video in whichchrominance signals are not resampled is attracting attention. Forexample, in the JVT (Joint Video Team), development for a new profilefor H.264 (advanced 4:4:4 profile) is underway.

Non-patent document 1: “Chrominance Signal Interpolation of YUV4:2:0Format Color Images”, Hiroshi Sugita, Akira Taguchi, IEICE (TheInstitute of Electronics, Information and Communication Engineers)Transactions, Vol. J88-A, No. 6, pp. 751-760, 2005

Non-patent document 2: “A Color Image Compression Scheme Based on theAdaptive Orthogonalized Transform - Accelerated by Sorted Dictionary”,Takashi Miura, Fumihiko Itagaki, Akihiko Yokoyama, Momoe Yamaguchi,IEICE (The Institute of Electronics, Information and CommunicationEngineers) Transactions, Vol. J85-D2, No. 11, pp. 1672-1682, November2002

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

Downsampling chrominance elements is an effective means of reducing theamount of code while suppressing any decrease in subjective imagequality. However, depending on the image content, in some cases furtherdownsampling is possible without this leading to deterioration insubjective image quality. In contrast, depending on the image content,in some cases the high image quality of a 4:4:4 format video is notachieved due to the chrominance elements in a frame all beingdownsampled uniformly.

The characteristics of an image signal changes for each frame, and thereare also localized changes within a frame. Visual sensitivity towardschrominance elements also changes in accordance with these changes. Forexample, in a frame (or in an area) having a small (i.e., a dark) pixelvalue, there is lowered visual sensitivity towards chrominance elementscompared with a frame (or an area) having a large (i.e., a bright) pixelvalue.

Because of this, by changing the downsampling ratio of chrominanceelements in accordance with the characteristics of each frame, or inaccordance with the localized characteristics within a frame, it may bepossible to efficiently reduce the amount of code while maintaining thesubjective image quality.

However, conventional downsampling of chrominance elements is performedat a uniform ratio over the entire frame, so that there is considerablescope for improvement in encoding efficiency.

The present invention was conceived in view of the above describedcircumstances, and it is an object thereof to establish novel videoencoding and decoding technology which efficiently reduces code amountsby introducing adaptive downsampling processing for each frame andintroducing adaptive downsampling processing for each area within eachframe.

Means for Solving the Problem

[1] Video Encoding Apparatus of the Present Invention which AdaptivelyChanges a Downsampling Ratio

[1-1] Structure for Making Adaptive Changes in Units of Frames

In order to achieve the above described object, the video encodingapparatus of the present invention is structured such that, when a videosignal made up of two or more signal elements is targeted for encoding,the video encoding apparatus is provided with: (1) a setting devicewhich sets a downsampling ratio for a specific signal element of a framein accordance with the characteristics inside the frame; and (2) agenerating device which generates a video signal which is targeted toinput to an encoder by downsampling a specific signal element inside aframe in accordance with the downsampling ratio set by the settingdevice.

Here, the video encoding method of the present invention which isrealized by the respective processing devices described above beingoperated can also be realized by means of a computer program. Thiscomputer program may be supplied by being recorded on a suitablecomputer-readable recording medium, or may be supplied via a network.When the present invention is to be implemented, the computer program isinstalled, and is then able to realize the present invention by beingoperated by a control device such as a CPU.

In view of the observation that, for example, when a comparison is madebetween a frame in which there is a large amount of transition and aframe in which there is a small amount of transition, there is arelatively low level of visual sensitivity in the former, and thus,compared with a frame having a small amount of transition, there is noreduction in subjective image quality in a frame having a large amountof transition even if large-scale downsampling is performed, in thevideo encoding apparatus of the present invention which is structured inthe manner described above, a video signal targeted for input into anencoder is generated, for example, by setting a downsampling ratio for aspecific signal element such as a chrominance signal in each frame inaccordance with the size of the amount of transition within the frame,and then downsampling the specific signal element in units of frames inaccordance with the set downsampling ratio.

In accordance with this structure, according to the video encodingapparatus of the present invention, it is possible to efficiently reducecode amounts while maintaining subjective image quality.

[1-2] Structure for Making Adaptive Changes in Units of Partial AreasWithin a Frame

In the above described video encoding apparatus, it is also possible forthe device which sets the downsampling ratio to have: a device whichdivides a frame into partial areas in accordance with localizedcharacteristics within the frame; and a device which sets downsamplingratios for specific signal elements in the partial areas in accordancewith the characteristics in each partial area, and for the video signaltargeted for encoding to be generated by downsampling the specificsignal element of the partial area in accordance with the setdownsampling ratio.

In view of the observation that, for example, when a comparison is madebetween a partial area in which there is a large amount of transitionand a partial area in which there is a small amount of transition, thereis a relatively low level of visual sensitivity in the former, and thus,compared with a partial area having a small amount of transition, thereis no reduction in subjective image quality in a partial area having alarge amount of transition even if large-scale downsampling isperformed, in the video encoding apparatus which is structured in such amanner, a video signal targeted for input into an encoder is generated,for example, by setting a downsampling ratio for a specific signalelement such as a chrominance signal in each partial area within a framein accordance with the size of the transition amount within the partialarea in the frame, and then downsampling the specific signal element inunits of partial areas within the frames in accordance with the setdownsampling ratio.

In the case of this structure as well, it is possible to efficientlyreduce code amounts while maintaining subjective image quality.

Video encoding apparatus and video decoding apparatus of the presentinvention which handle changes in an encoded block pattern generated asa result of the downsampling ratio being adaptively changed

The video encoding apparatus of the present invention has a basicstructure in which, when a video signal made up of two or more signalelements is taken as a target for encoding, video signals generated bydownsampling specific signal elements in accordance with a downsamplingratio set for each frame or set for each partial area within a frame areencoded.

In contrast, in a standard video encoding apparatus, a structure may beemployed in which, for each quantized signal element obtained byencoding, information is appended to each block of a specific sizeshowing whether or not a significant coefficient is contained in thatblock. If this type of structure is employed, for blocks which containno significant coefficient, it is possible to omit the appending of atransformation coefficient thereby enabling a reduction in the amount ofcode to be achieved.

However, in the video encoding apparatus of the present invention,because the downsampling ratio of a specific signal element is changedadaptively, the pattern formed by combining blocks for each signalelement does not become a fixed pattern.

Therefore, the present invention may further be provided with a devicewhich, for each quantized signal element obtained by encoding the videosignal targeted for encoding, when appending information to each blockof a specific size showing whether or not a significant coefficient iscontained in that block, appends information showing at which position agroup of blocks which share the specific signal element is located, tothe group of blocks.

In addition, the present invention is a video decoding apparatus whichdecodes encoded data of a video signal which has been generated by thisvideo encoding apparatus, wherein there are provided: a device which, bydecoding encoded data of the information showing at which position thegroup of blocks which share the specific signal element is located,determines whether or not the group of blocks targeted for decoding is agroup of blocks which share the specific signal element; and a devicewhich, for the group of blocks which the determination device hasdetermined to be a group of blocks which share the specific signalelement, by decoding encoded data of the specific signal element sharedby the group of blocks, decodes the downsampled signal element inaccordance with the downsampling ratio set on the encoding side.

Here, the video encoding method of the present invention and the videodecoding method of the present invention which are realized by therespective processing devices described above being operated can also berealized by means of a computer program. This computer program may besupplied by being recorded on a suitable computer-readable recordingmedium, or may be supplied via a network. When the present invention isto be implemented, the computer program is installed, and is then ableto realize the present invention by being operated by a control devicesuch as a CPU.

In the video decoding apparatus of the present invention which isstructured in this manner, information showing at which position thegroup of blocks which share the specific signal element is located isappended to the group of blocks.

In addition, the present invention is a video decoding apparatus whichdecodes encoded data of a video signal which has been generated by thisvideo encoding apparatus, wherein there are provided: a device which, bydecoding encoded data of information showing at which position the groupof blocks which share the specific signal element is located, determineswhether or not the group of blocks targeted for decoding is a group ofblocks which share the specific signal element; and a device which, fora group of blocks which the determination device has determined to be agroup of blocks which share the specific signal element, by decodingencoded data of the specific signal element shared by the group ofblocks, decodes the downsampled signal element in accordance with thedownsampling ratio set on the encoding side.

In accordance with this structure, according to the present invention,when a structure is employed in which a downsampling ratio is changedadaptively with frames or slices within frames being taken as units,even if the combination pattern of the blocks changes for each signalelement in accordance with this structure, it is possible for astructure which is used in a standard video encoding apparatus whichappends to blocks having a certain size information showing whether ornot a quantized signal contains a significant coefficient to be usedwithout this structure being changed.

[3] Video encoding apparatus and video decoding apparatus of the presentinvention which handle changes in the interpolation accuracy of areference frame which are generated as a result of the downsamplingratio being adaptively changed.

[3-1] When a Downsampling Ratio is Changed Adaptively in Units of Frames

The video encoding apparatus of the present invention has a basicstructure in which, when a video signal made up of two or more signalelements is taken as a target for encoding, video signals generated bydownsampling specific signal elements in accordance with a downsamplingratio set for each frame are encoded.

In contrast, in a video encoding apparatus, motion compensation having adecimal fraction pixel accuracy may be used for an inter-frameprediction. This is because highly accurate motion compensation can berealized if this type of structure is employed.

However, in the video encoding apparatus of the present invention,because the downsampling ratio of a specific signal element is changedadaptively in units of frames, the interpolation accuracy of a referenceframe which is required when motion compensation having a decimalfraction pixel accuracy is used for an inter-frame prediction does notbecome fixed.

Therefore, the above described video encoding apparatus of the presentinvention may further be provided with: a device which, when motioncompensation having a decimal-fraction pixel accuracy is used for aninter-frame prediction in the encoding of the video signal targeted forencoding, changes the interpolation accuracy of a downsampled signalelement in the reference frame in accordance with a ratio between thedownsampling ratio of the frame targeted for encoding and thedownsampling ratio of the reference frame; and a device which, based onthe changed interpolation accuracy, generates an interpolated image ofthe reference frame.

As a counterpart thereto, the present invention provides a videodecoding apparatus which decodes encoded data of a video signal whichhas been generated by the above described video encoding apparatus, andwhich is provided with: a device which, when motion compensation havinga decimal-fraction pixel accuracy is used for an inter-frame prediction,changes the interpolation accuracy of a downsampled signal element inthe reference frame in accordance with a ratio between the downsamplingratio of the frame targeted for decoding and the downsampling ratio ofthe reference frame; and a device which, based on the changedinterpolation accuracy, generates an interpolated image of the referenceframe.

Here, the video encoding method of the present invention and the videodecoding method of the present invention which are realized by therespective processing devices described above being operated can also berealized by means of a computer program. This computer program may besupplied by being recorded on a suitable computer-readable recordingmedium, or may be supplied via a network. When the present invention isto be implemented, the computer program is installed, and is then ableto realize the present invention by being operated by a control devicesuch as a CPU.

In the video encoding apparatus of the present invention which isstructured in this manner, the interpolation accuracy of a downsampledsignal element in a reference frame is changed in accordance with aratio between the downsampling ratio of a frame targeted for encodingand the downsampling ratio of the reference frame, and an interpolatedimage of the reference frame is generated based on the changedinterpolation accuracy.

Moreover, in the video decoding apparatus of the present invention whichis structured so as to correspond to this, the interpolation accuracy ofa downsampled signal element in a reference frame is changed inaccordance with a ratio between the downsampling ratio of a frametargeted for decoding and the downsampling ratio of the reference frame,and an interpolated image of the reference frame is generated based onthis changed interpolation accuracy.

In accordance with this structure, according to the present invention,when a structure is employed in which a downsampling ratio is changedadaptively in units of frames, even if the interpolation accuracy of thereference frame changes in accordance with this structure, it ispossible to generate an interpolated image of a reference frame whichrealizes the required motion compensation.

[3-2] When a Down Sampling Ratio is Changed Adaptively in Units ofPartial Areas Within a Frame

The video encoding apparatus of the present invention is also able toencode video signals generated by downsampling specific signal elementsmore finely in accordance with a downsampling ratio set for each partialarea within a frame.

As stated above, in a video encoding apparatus, motion compensationhaving a decimal fraction pixel accuracy may be used for an inter-frameprediction. This is because highly accurate motion compensation can berealized if this type of structure is employed.

However, in the video encoding apparatus of the present invention,because the downsampling ratio of a specific signal element is changedadaptively in units of partial areas within the frames, theinterpolation accuracy of a reference frame which is required whenmotion compensation having a decimal fraction pixel accuracy is used foran inter-frame prediction does not become fixed.

Therefore, the above described video encoding apparatus may further beprovided with: a device which, when motion compensation having a decimalfraction pixel accuracy is used for an inter-frame prediction in theencoding of the video signal targeted for encoding, changes theinterpolation accuracy of a downsampled signal element in a partial areawithin the reference frame in accordance with a ratio between thedownsampling ratio of each partial area within the frame targeted forencoding and the downsampling ratio of the partial area within thereference frame which is referred to when this partial area undergoesmotion compensation; and a device which, based on the changedinterpolation accuracy, generates an interpolated image of the partialarea within the reference frame.

As a counterpart thereto, the present invention provides a videodecoding apparatus which decodes encoded data of a video signal whichhas been generated using the video encoding method described above, andwhich includes: a device which, when motion compensation having adecimal fraction pixel accuracy is used for an inter-frame prediction,changes the interpolation accuracy of a downsampled signal element in apartial area within the reference frame in accordance with a ratiobetween the downsampling ratio of each partial area within the frametargeted for decoding and the downsampling ratio of the partial areawithin the reference frame which is referred to when this partial areaundergoes motion compensation; and a device which generates aninterpolated image of the partial area within the reference frame basedon the changed interpolation accuracy.

Here, the video encoding method of the present invention and the videodecoding method of the present invention which are realized by therespective processing devices described above being operated can also berealized by means of a computer program. This computer program may besupplied by being recorded on a suitable computer-readable recordingmedium, or may be supplied via a network. When the present invention isto be implemented, the computer program is installed, and is then ableto realize the present invention by being operated by a control devicesuch as a CPU.

In the video encoding apparatus of the present invention which isstructured in this manner, the interpolation accuracy of a downsampledsignal element in a partial area within a reference frame is changed inaccordance with a ratio between the downsampling ratio of a partial areawithin a frame targeted for encoding and the downsampling ratio of apartial area within the reference frame which is referred to when motioncompensation is performed on this partial area, and an interpolatedimage of the partial area within the reference frame is generated basedon the changed interpolation accuracy.

Moreover, in the video decoding apparatus of the present invention whichis structured so as to correspond to this, the interpolation accuracy ofa downsampled signal element in a partial area within a reference frameis changed in accordance with a ratio between the downsampling ratio ofa partial area within a frame targeted for decoding and the downsamplingratio of a partial area within the reference frame which is referred towhen motion compensation is performed on this partial area, and aninterpolated image of the partial area within the reference frame isgenerated based on the changed interpolation accuracy.

In accordance with this structure, according to the present invention,when a structure is employed in which a downsampling ratio is changedadaptively with in units of slices within a frame, even if theinterpolation accuracy of a reference frame changes in accordance withthis structure, it is possible to generate an interpolated image of areference frame which realizes the required motion compensation.

Effects of the Invention

According to the present invention, when encoding video, it is possibleto adaptively change the downsampling ratio of a specific signal elementin accordance with the characteristics of each frame or in accordancewith localized characteristics within a frame, and to consequentlyachieve an efficient code reduction in accordance with visualsensitivity.

Moreover, according to the present invention, even if the combinationpattern of blocks changes for each signal element as a result of adownsampling ratio being changed adaptively, it is possible for astructure, which is used in a standard video encoding apparatus whichappends to blocks having a certain size information showing whether ornot a quantized signal contains a significant coefficient, to be usedwithout this structure being changed.

In addition, according to the present invention, when motioncompensation having a decimal fraction pixel accuracy is used for aninter-frame prediction, even if there is a change in the interpolationaccuracy of a reference frame as a result of a downsampling ration beingchanged adaptively, it is possible to generate an interpolated image ofthe reference frame which realizes the required motion compensation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing downsampling processingaccording to the present invention.

FIG. 2 is an explanatory diagram showing downsampling processingaccording to the present invention.

FIG. 3 is an explanatory diagram showing downsampling processingaccording to the present invention.

FIG. 4 is an explanatory diagram showing downsampling processingaccording to the present invention.

FIG. 5 is an explanatory diagram of a CBP.

FIG. 6 is a diagram showing an example of a luminance element and adownsampled chrominance element.

FIG. 7A is an explanatory diagram of a chrominance shared block set.

FIG. 7B is also an explanatory diagram of a chrominance shared blockset.

FIG. 7C is also an explanatory diagram of a chrominance shared blockset.

FIG. 7D is also an explanatory diagram of a chrominance shared blockset.

FIG. 8 is an explanatory diagram showing interpolation accuracies of areference frame.

FIG. 9 is an embodiment of a flowchart showing encoding processingaccording to the present invention.

FIG. 10 is a continuation of the flowchart shown in FIG. 9.

FIG. 11 is an embodiment of a flowchart showing decoding processingaccording to the present invention.

FIG. 12 is a continuation of the flowchart shown in FIG. 11.

FIG. 13 is an embodiment of a video encoding apparatus of the presentinvention.

FIG. 14 is an embodiment of a video decoding apparatus of the presentinvention.

FIG. 15 is an embodiment of a reference frame interpolation imagegeneration apparatus of the present invention.

FIG. 16 is a diagram showing an example of the apparatus structure of avideo encoding apparatus.

FIG. 17 is a diagram showing an example of the apparatus structure of avideo decoding apparatus.

FIG. 18 is an embodiment of a flowchart showing reference frameinterpolation image generation processing of the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

-   100 Adaptive processing flag setting section-   101 Adaptive processing flag storage section-   102 Luminance signal reading section-   103 Luminance signal storage section-   104 Chrominance signal reading section-   105 Chrominance signal storage section-   106 Downsampling ratio setting section-   107 Downsampling ratio storage section-   108 Chrominance signal downsampling processing section-   109 Chrominance signal storage section-   110 Chrominance-possessing macroblock position information setting    section-   110-1 Sequence adaptive processing section-   110-2 Slice adaptive processing section-   110-3 Chrominance adaptive processing section-   111 Encoded stream generation section-   111-1 Header information encoding section-   111-2 Header information encoded stream storage section-   111-3 Luminance signal—chrominance signal encoding processing    section-   111-4 Luminance signal—chrominance signal encoded stream storage    section-   111-5 Encoded stream multiplexing section-   112 Encoded stream storage section-   113 Final slice determination section

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail inaccordance with the embodiments.

In the present invention, novel video encoding and decoding technologyis realized which efficiently reduces code amounts by introducingadaptive downsampling processing for each frame and by introducingadaptive downsampling processing for each area within each frame.

Required functions in an encoder and a decoder in order to introducethis type of adaptive downsampling processing are described below.

Note that in the example given below, the chrominance element in a colorspace formed by a luminance signal (Y) and two chrominance elements (Cb,Cr) is the downsampling target, however, targets of the presentinvention are not limited to these color space signals.

For example, the present invention can also be applied in the same wayto three elements obtained by performing any color transformation on4:4:4 format video signals. Furthermore, the number of elements on whichadaptive downsampling processing is performed is not limited to anyspecific number. For example, when a first principal element, a secondprincipal element, and a third principal element are obtained using KLtransformation for the color transformation, then any one of theseelements may be the downsampling target. Moreover, another example mightbe for any two of these three elements (for example, the secondprincipal element and the third principal element) to be thedownsampling target. Furthermore, it is naturally also possible for allthree of these elements to be the downsampling target.

Furthermore, the present invention can also be applied in the same waywhen a signal other than a color space signal is the downsamplingtarget.

[1] Frame Unit Adaptive Processing

In a first aspect of the present invention, a frame downsampling ratiois adaptively changed for a chrominance element in accordance with thecharacteristics inside the frame.

The frame downsampling ratio is r_(f)[t](<1) as defined by the followingformula:r _(f) [t]=W′[t]/W[t]=H′[t]/H [t]when the size of a t^(th) frame of a 4:4:4 format video is taken asW[t]×H[t][pixels], and when the frame size of a chrominance elementafter downsampling is taken as W′[t]×H′[t][pixels].

The example shown in FIG. 1 is an example in which the framedownsampling ratio is set to 1/2, 1/4, 1/4 in sequence from the leadingframe.

Here, the frame downsampling ratios of the two chrominance elements(i.e., the Cb and Cr elements) are the same value.

Here, it is also possible to set frame downsampling ratios independentlyfor the two chrominance elements (i.e., the Cb and Cr elements).

FIG. 2 shows an example of this. Here, in sequence from the leadingframe, the frame downsampling ratio for the Cb element is set to 1/2,1/4, 1/2, while the frame downsampling ratio for the Cr element is setto 1/2, 1/2, 1/4.

The method used to set the downsampling ratio for each frame is providedfrom an external mechanism which is provided separately from themechanism performing the downsampling.

For example, it is possible to set a downsampling ratio for each framein accordance with the size of the amount of transition (i.e., change)within a frame.

When a frame having a large amount of transition is compared to a framehaving a small amount of transition, then there is a relatively lowlevel of visual sensitivity in the former. Because of this, in a framehaving a large amount of transition, it is possible to set a smalldownsampling ratio so that large downsampling is conducted compared witha frame having a small amount of transition.

Accordingly, in this case, the mechanism which sets the downsamplingratio detects the size of the transition amount for a frame, determinesthe downsampling ratio which corresponds to this size, and then gives acommand to the mechanism performing the downsampling to downsample thisframe in accordance with the determined downsampling ratio.

[2] Slice Unit Adaptive Processing

In a second aspect of the present invention, the slice downsamplingratio is adaptively changed for chrominance elements in accordance withthe characteristics within a frame.

The slice downsampling ratio is r_(s)[t,i](<1) as defined by thefollowing formula:r _(s) [t,i]=w′[t,i]/w[t,i]=h′[t,i]/h[t,i]when a t^(th) frame of a 4:4:4 format video chrominance element isdivided into several partial areas (referred to as ‘slices’), and thesize of the i^(th) slice is taken as w[t,i]×h[t,i][pixels], and when thesize of the i^(th) slice after downsampling is taken asw′[t,i]×h′[t,i][pixels].

Here, I=0, 1, . . . , I−1, wherein I is the total number of sliceswithin a frame.

The top row in FIG. 3 shows an example of a case in which chrominanceelement frames are divided into two slices. Hereinafter, forconvenience, of these two slices, the top slice is referred to as thefirst slice, while the bottom slice is referred to as the second slice.

In this example, as is shown in the bottom row in FIG. 3, the slicedownsampling ratio of the first slice is set to 1/2 and the slicedownsampling ratio of the second slice is set to 1/4. Here, the slicedownsampling ratios of the two chrominance elements (i.e., the Cb and Crelements) are the same value.

Here, it is also possible to set slice downsampling ratios independentlyfor the two chrominance elements (i.e., the Cb and Cr elements).

FIG. 4 shows an example of this. As is shown in the top row, in the sameslice division settings as in FIG. 3, the slice downsampling ratios forthe first slice and the second slice of the Cb element are setrespectively to 1/2 and 1/4, while the slice downsampling ratios for thefirst slice and the second slice of the Cr element are set respectivelyto 1/4 and 1/2.

While the above described frame downsampling ratios enable thedownsampling ratio to be changed for each frame, these slicedownsampling ratios further enable the downsampling ratio to be changedin localized areas within a frame.

The method used to divide a frame into slices and the method used to setthe slice downsampling ratio for each slice are provided from anexternal mechanism which is provided separately from the mechanismperforming the slice downsampling.

For example, it is possible to divide a frame into slice areas inaccordance with the size of the amount of transition (i.e., change).When a slice having a large amount of transition is compared to a slicehaving a small amount of transition, then there is a relatively lowlevel of visual sensitivity in the former. Because of this, in a slicehaving a large amount of transition, it is possible to set a small slicedownsampling ratio so that large-scale slice downsampling is conductedcompared with a slice having a small amount of transition.

Accordingly, in this case, the mechanism which sets the slicedownsampling ratios detects the size of the transition amount for eachsmall area within a frame, divides the frame into slice areas based onthis and also determines for each slice the slice downsampling ratiowhich corresponds to the transition amount within that slice, and thengives a command to the mechanism performing the slice downsampling toperform slice downsampling on the relevant slice in accordance with thedetermined slice downsampling ratio.

[3] Encoding Block Pattern

Next, a description will be given of bit allocation relating to CBP(described below) when the adaptive frame downsampling and slicedownsampling of the present invention are performed.

In H.264, the luminance element of 16×16 [pixels] and the chrominanceelement of two 8×8 [pixels] are known as a macroblock, and atransformation coefficient is transformed into an encoded stream inunits of these macroblocks.

At this time, as is shown in FIG. 5, six bits of information indicatingwhether or not the transformation coefficient within each sub blockincludes a significant coefficient (i.e., a non-zero coefficient) areappended to six sub blocks which are formed by four 8×8 [pixels]luminance element sub blocks and two chrominance element sub blocks ofidentical size. This information is called a “Coded Block Pattern”(CBP).

Note that the numerals inside the sub blocks in FIG. 5 show the positionof the bit within the CBP for each sub block.

Encoding information for a transformation coefficient other than a CBPis not appended to sub blocks in which the corresponding bit within theCBP is 0. This is because this processing is intended to reduce theamount of code. In contrast, transformation coefficient information isappended to sub blocks in which the bit is 1.

In H.264, a reduction in the amount of code is achieved using this typeof CBP, and if the adaptive frame downsampling and slice downsampling ofthe present invention are to be performed, it is necessary to build astructure which enables this CBP to be used.

Accordingly, in the present invention, when the downsampling ratio ofthe chrominance elements is variable, then the bit allocation for thisCBP is modified.

Next, a description will be given of the bit allocation for the CBP ofthe present invention.

As an example, a case is considered in which the size of the luminanceelements in a macroblock is taken as 16×16, and the frame downsamplingratio (or slice downsampling ratio) is taken as r (<1/2).

In this case, two types of macroblock having different structuralelements are generated.

Namely, these two types of macroblock are a macroblock which is formedby six sub blocks which are formed by four 8×8 [pixels] luminanceelement sub blocks and two chrominance element sub blocks of identicalsize, and a macroblock which is formed only by four 8×8 [pixels]luminance element sub blocks.

In the case of the former, in the same way as in the conventionaltechnology, the CBP is represented by 6 bits. In contrast, in the caseof the latter, the CBP is represented by 4 bits.

FIG. 6 shows an example in which r=1/4. In this case, four combinationsof chrominance elements and luminance elements may be considered asshown in FIG. 7A through FIG. 7D. Because of this, information showingat which position a macroblock possessing a chrominance element islocated is required.

A group of pixels which is formed by chrominance elements and by theluminance elements which share these chrominance elements is called achrominance shared block set. Namely, in any one of FIG. 7A through FIG.7D, the chrominance elements (i.e., the portion indicated by the shadingin the drawing) belonging to a macroblock is shared by the other threemacroblocks (i.e., those having only luminance elements).

Moreover, in a chrominance shared block set, the position of amacroblock possessing chrominance elements is called achrominance-possessing macroblock position.

A method of representing this chrominance-possessing macroblock positionwill now be described in relation to the method of representation usedby the present invention.

[3-1] Third Aspect of the Present Invention

This is a method in which a chrominance-possessing macroblock positionis determined in advance.

For example, in this example it is determined that only the macroblockwhich is closest to the beginning in the scanning sequence possesses achrominance element. If this position is shared by an encoder and adecoder in advance, then decoding can be performed in the decoderwithout any additional information.

[3-2] Fourth Aspect of the Present Invention

This is a method in which a chrominance-possessing macroblock positionis set on the encoder side for each sequence (i.e., an entire compressedvideo). In this case, the chrominance-possessing macroblock position isset as a fixed value within the sequence. In conjunction with this, itis necessary to append the information below in an encoded stream asencoding information.

Firstly, a flag (chroma-position-sequence-adaptive) indicating whetheror not this adaptive processing is to be applied is appended to anencoded stream as header information.

This flag is set to 1 when this adaptive processing is being performed,and is set to 0 when this adaptive processing is not being performed.

Furthermore, because the chrominance-possessing macroblock positionchanges for each sequence, a chrominance-possessing macroblock positionis appended to an encoded stream as header information about thesequence for each sequence. This processing is called sequence adaptivechrominance-possessing macroblock position varying processing.

[3-3] Fifth Aspect of the Present Invention

This is a method in which a chrominance-possessing macroblock positionis set on the encoder side for each slice.

Here, as is described above, the term ‘slice’ indicates a partial areaobtained by dividing the interior of each frame. The specific divisionmethod is provided from a mechanism for determining slice divisionswhich is provided externally.

In this case, the chrominance-possessing macroblock position is set as afixed value within the slice. In conjunction with this, it is necessaryto append the information below in an encoded stream as encodinginformation.

Firstly, a flag (chroma-position-slice-adaptive) indicating whether ornot this adaptive processing is to be applied is appended to an encodedstream as header information. This flag is set to 1 when this adaptiveprocessing is being performed, and is set to 0 when this adaptiveprocessing is not being performed.

Furthermore, because the chrominance-possessing macroblock positionchanges for each slice, it is necessary to append achrominance-possessing macroblock position to an encoded stream asheader information about the slice to each slice. This processing iscalled slice adaptive chrominance-possessing macroblock position varyingprocessing.

[3-4] Sixth Aspect of the Present Invention

This is a method in which a chrominance-possessing macroblock positionis set on the encoder side for each chrominance shared block set. Inthis case, it is possible to change the chrominance-possessingmacroblock position for each chrominance shared block set. Inconjunction with this, it is necessary to append the information belowto an encoded stream as encoding information.

Firstly, a flag (chroma-position-MBs-adaptive) indicating whether or notthis adaptive processing is to be applied is appended to an encodedstream as header information. This flag is set to 1 when this adaptiveprocessing is being performed, and is set to 0 when this adaptiveprocessing is not being performed.

Furthermore, because the chrominance-possessing macroblock positionchanges for each chrominance shared block set, it is necessary to appenda chrominance-possessing macroblock position to an encoded stream asheader information about the chrominance shared block set to eachchrominance shared block set. This processing is called chrominanceshared block set adaptive chrominance-possessing macroblock positionvarying processing.

[4] Motion Vector Encoding Format

Next, a description will be given of a motion vector encoding formatwhen the adaptive frame downsampling and slice downsampling of thepresent invention are performed.

Within a chrominance shared block set, luminance elements andchrominance elements are taken as sharing the same motion vectorinformation. However, for the chrominance, it is necessary to performscaling on the motion vector in accordance with the downsampling ratio.

For example, when the motion vector appended to the encoded stream isv=(4, 8), and the downsampling ratio is 1/4, then the motion vector ofthe luminance element is v=(4, 8), and the motion vector of thechrominance element is (1/4)×v =(1, 2).

[4-1] Seventh Aspect of the Present Invention

The interpolation accuracy of the chrominance element of a referenceframe is changed based on Formula (1) given below in accordance with theframe downsampling ratio of a frame targeted for encoding (i.e., at^(th) frame) and a reference frame (i.e., a t−1^(th) frame), and, basedon this, an interpolated image of the chrominance element of thereference frame is generated.

Note that in the formula given below, r_(f)[t] is the frame downsamplingratio of the frame targeted for encoding, while r_(f)[t−1] is the framedownsampling ratio of the reference frame.

When a motion estimation accuracy (i.e., a motion compensation accuracy)of the luminance element of the t^(th) frame is 1/M, then theinterpolation accuracy A of the chrominance element of the referenceframe is determined in accordance with the following Formula (1)A=(r _(f) [t−1]/r _(f) [t])×(1/M)  (1)

For example, when the motion estimation of the luminance element of theframe targeted for encoding has an accuracy of 1/4 pixels, and when thedownsampling ratio of the chrominance element of the frame targeted forencoding is 1/4, and when the downsampling ratio of the chrominanceelement of the reference frame is 1/2, then, in accordance with Formula(1), the motion interpolation accuracy A of the chrominance element ofthe reference frame is found to be an accuracy of 1/2 pixels.A=(1/2)÷(1/4)×(1/4)=1/2

This is for the following reason. When the downsampling ratio of thechrominance element of the frame targeted for encoding is 1/4, then themotion vector of the chrominance element which is used is obtained byscaling the motion vector of the luminance element by 1/4. By performingthis scaling, information of less than one pixel of the motion vector isdiscarded, and the motion estimation accuracy of the chrominance elementof the frame targeted for encoding is made into an accuracy of 1 pixel.

In contrast, the downsampling ratio of the reference frame is 1/2, andis the resolution of half the luminance signal. Because of this, inorder to obtain information about the integer pixel position requiredfor the chrominance element of the frame targeted for encoding, it isnecessary to obtain the information for a position which corresponds toa 1/2 pixel in the reference frame by interpolation.

FIG. 8 shows the collected results of the interpolation accuracy (i.e.,the interpolation accuracy A determined using Formula (1)) of thereference frame when the motion estimation accuracy of the luminanceelement is an accuracy of 1/4 pixels, and the downsampling ratios of theframe targeted for encoding and of the reference frame are changed by 1,1/2, 1/4.

Here, the motion estimation accuracy required for the (chrominanceelement of the) frame targeted for encoding shown in FIG. 8 isdetermined in accordance with the following formula.(1/r_(f)[t])×(1/M)[4-2] Eighth Aspect of the Present Invention

The interpolation accuracy in a slice of a chrominance element of thereference frame is changed based on Formula (2) given below inaccordance with the slice downsampling ratios of the frame targeted forencoding (i.e. the t^(th) frame) and the reference frame (i.e., thet−1^(th) frame). Based on this, an interpolated image of the slice ofthe chrominance element of the reference frame is generated.

Note that in the formula given below, a case is considered in which ablock within an i^(th) slice of the frame targeted for encoding takes anarea within a j^(th) slice of the reference frame as a reference signalduring motion compensation.

At this time, the slice downsampling ratio of the i^(th) slice of theframe targeted for encoding is r_(s)[t,i], while the slice downsamplingratio of the j^(th) slice of the reference frame is r_(s)[t−1, j].

When a motion estimation accuracy of the luminance element of the t^(th)frame is 1/M, then the interpolation accuracy A [j] of the j^(th) sliceof the chrominance element of the reference frame is determined inaccordance with the following Formula (2)A[j]=(r _(s) [t−1, j]/r _(s) [t,i])×(1/M)  (2)Embodiments

Next, the present invention will be described in detail in accordancewith the following embodiments.

FIG. 9 and FIG. 10 show an embodiment of a flowchart showing theencoding processing executed by the present invention. The encodingprocessing executed by the present invention will be described in detailin accordance with this flowchart.

Step S100: The flags relating to the setting of thechrominance-possessing macroblock position information (i.e.,chroma-position-sequence-adaptive, chroma-position-slice-adaptive, andchroma-position-MBs-adaptive) are read, and written in registers. Thisis to determine whether the above described sequence adaptivechrominance-possessing macroblock position varying processing, or theabove described slice adaptive chrominance-possessing macroblockposition varying processing, or the above described chrominance shardblock set adaptive chrominance-possessing macroblock position varyingprocessing is to be used.

Step S101: A determination is made as to whether or not achroma-position-sequence-adaptive flag which relates to the setting ofthe chrominance-possessing macroblock position information is set to 1,and if the result of this determination is a true value (namely, Y(Yes)), the routine moves to step S102. If, however, the determinationresult is N (No), the routine moves to step S103.

Step S102: Processing to set the chrominance-possessing macroblockposition information is performed, and the chrominance-possessingmacroblock position information is written in the register.

Here, the chrominance-possessing macroblock position information whichhas been set is used throughout the encoding of the entire sequence.This corresponds to the fourth aspect described above.

The specific setting values are provided from an externally providedmechanism which performs the setting of the chrominance-possessingmacroblock position information. For example, a method may be used inwhich, taking the weighted sum of encoding distortion and code amount asa cost function, the chrominance-possessing macroblock positioninformation is set such that this cost function is minimized.

Step S103: A determination is made as to whether or not thechroma-position-sequence-adaptive flag which relates to the setting ofthe chrominance-possessing macroblock position information is set to 1,and if the result of this determination is a true value, the routinemoves to step S106. If, however, the determination result is not a truevalue, the routine moves to step S104.

Step S104: A determination is made as to whether or not achroma-position-slice-adaptive flag which relates to the setting of thechrominance-possessing macroblock position information is set to 1, andif the result of this determination is a true value, the routine movesto step S105. If, however, the determination result is not a true value,the routine moves to step S106.

Step S105: Processing to set the chrominance-possessing macroblockposition information is performed, and the chrominance-possessingmacroblock position information is written in the register.

Here, the chrominance-possessing macroblock position information whichhas been set is used throughout the encoding in a slice. Thiscorresponds to the fifth aspect described above.

The specific setting values are provided from an externally providedmechanism which performs the setting of the chrominance-possessingmacroblock position information. For example, a method may be used inwhich, taking the weighted sum of encoding distortion and code amount asa cost function, the chrominance-possessing macroblock positioninformation is set such that this cost function is minimized.

Step S106: The luminance signal and chrominance signal of the slicecurrently being observed is read and written in the register.

Step S107: A downsampling ratio is set for the chrominance signal. Here,the set downsampling ratio is the slice downsampling ratio.

The specific setting values are provided from an externally providedmechanism which performs the setting of the slice downsampling ratio.For example, in a slice having a low luminance value, a method may beemployed in which a small downsampling ratio is set such thatlarge-scale downsampling is performed on the chrominance element.

Step S108: The set downsampling ratio and the chrominance signal writtenin the register are input, and downsampling processing of thechrominance element is performed in accordance with the inputdownsampling ratio. The chrominance element after the downsampling iswritten in the register.

The coefficient of the filter used in the downsampling is provided froman externally provided mechanism which determines filter coefficients inaccordance with the downsampling ratio.

Step S109: A determination is made as to whether or not at least one ofthe chroma-position-sequence-adaptive flag or thechroma-position-slice-adaptive flag relating to the setting of thechrominance-possessing macroblock position information is set to 1, andif the result of this determination is a true value, the routine movesto step S112. If, however, the determination result is not a true value,the routine moves to step S110.

Step S110: A determination is made as to whether or not achroma-position-MBs-adaptive flag which relates to the setting of thechrominance-possessing macroblock position information is set to 1, andif the result of this determination is a true value, the routine movesto step S111. If, however, the determination result is not a true value,the routine moves to step S112.

Step S111: Processing to set the chrominance-possessing macroblockposition information is performed, and the chrominance-possessingmacroblock position information is written in the register. Here, thechrominance-possessing macroblock position information which has beenset is used throughout the encoding of a chrominance shared block set.This corresponds to the sixth aspect described above.

The specific setting values are provided from an externally providedmechanism which performs the setting of the chrominance-possessingmacroblock position information. For example, a method may be used inwhich, taking the weighted sum of encoding distortion and code amount asa cost function, the chrominance-possessing macroblock positioninformation is set such that this cost function is minimized.

Step S112: The set chrominance-possessing macroblock positioninformation and downsampling ratio are read and written in theregisters.

Step S113: The chrominance-possessing macroblock position informationwhich was written in the register in step S112 is input, and adetermination is made as to whether or not the macroblock targeted forencoding is in the chrominance-possessing macroblock position. A true orfalse value is then output as the result of this determination.

If the output is a true value, the routine moves to step S114. If theoutput is a false value the routine moves to step S115.

Step S114: Using the chrominance signal as an input, encoding processingis performed, and the encoding result is written in the register.

The specific encoding method differs depending on the encoding algorithmwhich is used. For example, in H.264 and MPEG-2, the encoding methodinvolves motion compensation and discrete cosine transform.

Step S115: Header information generated in the encoding is written inthe register.

The specific symbols and the like targeted for encoding differ dependingon the encoding algorithm which is used. For example, in the case ofH.264, synchronous code forms the symbols targeted for encoding (here,symbol refers to structural units of information).

Step S116: Using the luminance signal as an input, encoding processingis performed, and the encoding result is written in the register.

The specific encoding method differs depending on the encoding algorithmwhich is used. For example, in H.264 and MPEG-2, the encoding methodinvolves motion compensation and discrete cosine transform.

Step S117 and step S118: The processing of step S113 through step S116is repeated for all of the macroblocks within the chrominance sharedblock set.

Step S119: The processing of step S109 through step S118 is repeated forall of the chrominance shared block sets within the slice.

Step S120: The processing of step S103 through step S119 is repeated forall of the slices within the sequence.

In this manner, in the present embodiment, luminance elements andchrominance elements are encoded while slice downsampling is performedfor the chrominance elements in accordance with the slice downsamplingratio which has been set adaptively. In addition, processing isperformed in order to set chrominance-possessing macroblock positioninformation, which shows which macroblocks possess chrominance elements,and to encode this as header information.

FIG. 11 and FIG. 12 illustrate an embodiment of a flowchart showingdecoding processing according to the present invention in which encodeddata generated in the encoding processing of the flowchart shown in FIG.9 and FIG. 10 is targeted for decoding. The decoding processing executedby the present invention will now be described in detail in accordancewith this flowchart.

Step S200: Taking an encoded stream as an input, processing to decode aflag (chroma-position-sequence-adaptive, chroma-position-slice-adaptive,chroma-position-MBs-adaptive) relating to the setting of thechrominance-possessing macroblock position information is executed, andthe value of the obtained flag is written in the register.

Step S201: A determination is made as to whether or not thechroma-position-sequence-adaptive flag relating to the setting of thechrominance-possessing macroblock position information is set to 1, andif the result of this determination is a true value, the routine movesto step S202. If, however, the determination result is not a true value,the routine moves to step S203.

Step S202: Taking the encoded stream as an input, processing to decodethe chrominance-possessing macroblock position information is performed,and the value of the chrominance-possessing macroblock positioninformation thus obtained is written in the register. This is theprocessing performed when the above described fourth aspect is used.

Step S203: Taking the encoded stream as an input, processing to decodethe downsampling ratio is performed, and the value of the downsamplingratio is written in the register.

Step S204: A determination is made as to whether or not thechroma-position-slice-adaptive flag relating to the setting of thechrominance-possessing macroblock position information is set to 1, andif the result of this determination is a true value, the routine movesto step S205. If, however, the determination result is not a true value,the routine moves to step S206.

Step S205: Taking the encoded stream as an input, processing to decodethe chrominance-possessing macroblock position information is performed,and the value of the chrominance-possessing macroblock positioninformation thus obtained is written in the register. This is theprocessing performed when the above described fifth aspect is used.

Step S206: A determination is made as to whether or not thechroma-position-MBs-adaptive flag relating to the setting of thechrominance-possessing macroblock position information is set to 1, andif the result of this determination is a true value, the routine movesto step S207. If, however, the determination result is not a true value,the routine moves to step S208.

Step S207: Taking the encoded stream as an input, processing to decodethe chrominance-possessing macroblock position information is performed,and the value of the chrominance-possessing macroblock positioninformation thus obtained is written in the register. This is theprocessing performed when the above described sixth aspect is used.

Step S208: Taking the chrominance-possessing macroblock positioninformation as an input, a determination is made as to whether or notthe macroblock targeted for encoding is in a chrominance-possessingmacroblock position, and a true or false value is output as the resultof this determination. If the output is a true value, the routine movesto step S209. If the output is a false value the routine moves to stepS210.

Step S209: Taking the encoded stream as an input, processing to decodethe chrominance element is performed, and the decoding result is writtenin the register.

Step S210: Taking the encoded stream as an input, processing to decodethe luminance element is performed, and the decoding result is writtenin the register.

Step S211 and step S212: The processing of step S208 through step S210is repeated for all of the macroblocks within the chrominance sharedblock set.

Step S213: Taking the downsampling ratio which was decoded in step S203as an input, processing to convert the downsampling ratio into anupsampling ratio is performed, and the calculated value is written inthe register.

This upsampling ratio is an inverse number of the downsampling ratio.

Step S214: Taking the upsampling ratio obtained in step S213 and thechrominance element decoded in step S209 as inputs, upsamplingprocessing is performed on this chrominance element, and the processedchrominance element is written in the register.

Step S215: The processing of step S206 through step S214 is repeated forall of the chrominance shared block sets within a slice.

Step S216: The processing of step S203 through step S215 is repeated forall of the slices within a sequence.

In this manner, in the present invention, taking the encoded datagenerated by the encoding processing shown in the flowcharts in FIG. 9and FIG. 10 as the target for decoding, processing is performed todecode a video signal.

FIG. 13 shows an embodiment of a video encoding apparatus of the presentinvention which executes the encoding processing shown in the flowchartsin FIG. 9 and FIG. 10. A description of the apparatus structure of thisvideo encoding apparatus will now be given in accordance with FIG. 13.

Here, in FIG. 13, 100 is an adaptive processing flag setting section,101 is an adaptive processing flag storage section, 102 is a luminancesignal reading section, 103 is a luminance signal storage section, 104is a chrominance signal reading section, 105 is a chrominance signalstorage section, 106 is a downsampling ratio setting section, 107 is adownsampling ratio storage section, 108 is a chrominance signaldownsampling processing section, 109 is a chrominance signal storagesection, 110 is a chrominance-possessing macroblock position informationsetting section, 111 is an encoded stream generation section, 112 is anencoded stream storage section, and 113 is a final slice determinationsection.

This chrominance-possessing macroblock position information settingsection 110 is provided with a sequence adaptive processing section110-1, a slice adaptive processing section 110-2, and a chrominanceadaptive processing section 110-3.

Moreover, the encoded stream generation section 111 is provided with aheader information encoding section 111-1, a header information encodedstream storage section 111-2, a luminance signal—chrominance signalencoding processing section 111-3, a luminance signal—chrominance signalencoded stream storage section 111-4, and an encoded stream multiplexingsection 111-5.

Next, a description of the processing executed by each of theseprocessing sections will be given.

The adaptive processing flag setting section 100 reads the flags(chroma-position-sequence-adaptive, chroma-position-slice-adaptive,chroma-position-MBs-adaptive) relating to the setting of thechrominance-possessing macroblock position information, and writes themin the adaptive processing flag storage section 101.

The luminance signal reading section 102 reads luminance signals andwrites them in the luminance signal storage section 103. The units inwhich the luminance signals are read are slices in the presentembodiment; however, it is also possible to read them in larger units orsmaller units than slices.

The chrominance signal reading section 104 reads chrominance signals andwrites them in the chrominance signal storage section 105. The units inwhich the chrominance signals are read are slices in the presentembodiment, however, it is also possible to read them in larger units orsmaller units than slices.

The downsampling ratio setting section 106 performs processing to setdownsampling ratios, and writes the set values in the downsampling ratiostorage section 107. Here, the set downsampling ratios are slicedownsampling ratios.

The specific setting values are provided from an externally providedmechanism which performs the setting of the slice downsampling ratio.For example, in a slice having a low luminance value, a method may beemployed in which a small downsampling ratio is set such thatlarge-scale downsampling is performed on the chrominance element.

The chrominance signal downsampling processing section 108 takeschrominance signals read from the chrominance signal storage section 105and downsampling ratios read from the downsampling ratio storage section107 as inputs, and performs downsampling processing on a chrominanceelement in accordance with an input downsampling ratio, and thedownsampled chrominance element is written in the chrominance signalstorage section 109.

The coefficient of the filter used in the downsampling is provided froman externally provided mechanism which determines filter coefficients inaccordance with the downsampling ratio.

The chrominance-possessing macroblock position information settingsection 110 takes luminance signals read from the luminance signalstorage section 103, pre-downsampling chrominance signals read from thechrominance signal storage section 105, downsampled chrominance signalsread from the chrominance signal storage section 109, and downsamplingratios read from the downsampling ratio storage section 107 as inputs,and performs processing to set chrominance-possessing macroblockposition information. The obtained chrominance-possessing macroblockposition information is given to the encoded stream generation section111.

Moreover, based on the obtained chrominance-possessing macroblockposition information, the downsampled chrominance signals output fromthe chrominance signal storage section 109 are given to the encodedstream generation section 111.

Note that the setting of the chrominance-possessing macroblock positioninformation is described in detail in the flowcharts in FIG. 9 and FIG.10.

The encoded stream generation section 111 performs the processing of theheader information encoding section 111-1 through the encoded streammultiplexing section 111-5 described below.

The header information encoding section 111-1 reads thechrominance-possessing macroblock position information and thedownsampling ratio as inputs together with other header information, andperforms encoding processing. It then writes the obtained encoded streamin the header information encoded stream storage section 111-2.

The luminance signal—chrominance signal encoding processing section111-3 takes luminance signals read from the luminance signal storagesection 103 and downsampled chrominance signals read from thechrominance signal storage section 109 as inputs, and performs encodingprocessing. It then writes the obtained encoded stream in the luminancesignal—chrominance signal encoded stream storage section 111-4.

The encoded stream multiplexing section 111-5 takes the encoded streamof the header information read from the header information encodedstream storage section 111-2 and the encoded stream for the luminanceelement and chrominance element read from the luminancesignal—chrominance signal encoded stream storage section 111-4 asinputs, and multiplexes the two streams. It then writes the multiplexedstream in the encoded stream storage section 112.

The final slice determination section 113 determines whether or not theslice currently undergoing processing is the final slice in order toperform the above described processing for all slices within a sequence.It also outputs encoded streams stored in the encoded stream storagesection 112.

In this manner, the video encoding apparatus shown in FIG. 13 executesthe encoding processing shown in the flowcharts in FIG. 9 and FIG. 10 inaccordance with the apparatus structure shown in FIG. 13.

FIG. 14 shows an embodiment of a video decoding apparatus of the presentinvention which executes the decoding processing shown in the flowchartsin FIG. 11 and FIG. 12. A description of the apparatus structure of thisvideo decoding apparatus will now be given in accordance with FIG. 14.

Here, in FIG. 14, 200 is a header information decoding section, 201 is aluminance signal decoding processing section, 202 is a luminance signalstorage section, 203 is a downsampling ratio storage section, 204 is achrominance-possessing macroblock position information storage section,205 is a chrominance-possessing macroblock position informationdetermination section, 206 is a chrominance signal decoding processingsection, 207 is a chrominance signal storage section, 208 is achrominance signal upsampling processing section, and 209 is achrominance signal storage section.

This header information decoding section 200 is provided with adownsampling ratio decoding processing section 200-1, and with achrominance-possessing macroblock position information decodingprocessing section 200-2.

The chrominance-possessing macroblock position information decodingprocessing section 200-2 is provided with a sequence adaptive processingflag detection section 200-3, a sequence adaptive processing flagstorage section 200-4, a sequence adaptive processing determinationsection 200-5, a slice adaptive processing flag detection section 200-6,a slice adaptive processing flag storage section 200-7, a slice adaptiveprocessing determination section 200-8, a chrominance shared block setadaptive processing flag detection section 200-9, a chrominance sharedblock set adaptive processing flag storage section 200-10, a chrominanceshared block set adaptive processing determination section 200-11, astream storage section 200-12, and a chrominance-possessing macroblockposition information decoding section 200-13.

Next, the processing executed by each of these processing sections willbe described.

The header information decoding section 200 performs processing todecode header information taking the encoded stream as an input. Thetargets of the decoding are the downsampling ratio, thechrominance-possessing macroblock position information, and other headerinformation (i.e., signals which are not targeted for decodingprocessing by the luminance signal decoding processing section 201 andthe chrominance signal decoding processing section 206).

The downsampling ratio decoding processing section 200-1 performsprocessing to decode the downsampling ratio using the encoded stream asan input, and then writes the downsampling ratio in the downsamplingratio storage section 203.

The chrominance-possessing macroblock position information decodingprocessing section 200-2 performs processing to decode thechrominance-possessing macroblock position information using the encodedstream as an input, and then writes the chrominance-possessingmacroblock position information in the chrominance-possessing macroblockposition information storage section 204.

The processing performed by the chrominance-possessing macroblockposition information decoding processing section 200-2 includes theprocessing performed by the sequence adaptive processing flag detectionsection 200-3 through the processing performed by thechrominance-possessing macroblock position information decoding section200-13 described below.

The sequence adaptive processing flag detection section 200-3 detectsthe chroma-position-sequence-adaptive flag which relates to the settingof the chrominance-possessing macroblock position information using theencoded stream as an input, and then writes the obtained flag value inthe sequence adaptive processing flag storage section 200-4.

The sequence adaptive processing determination section 200-5 takes thevalue of the chroma-position-sequence-adaptive read from the sequenceadaptive processing flag storage section 200-4 as an input, performsdetermination processing to determine whether or not the value is 1, andoutputs a true or false value. If the output is a true value, it writesin the stream storage section 200-12 the encoded stream of thechrominance-possessing macroblock position information which follows onfrom the chroma-position-sequence-adaptive flag in the encoded stream.

If, however, the output is not a true value, then the encoded stream isinput into the slice adaptive processing flag detection section 200-6.

The slice adaptive processing flag detection section 200-6 detects thechroma-position-slice-adaptive flag which relates to the setting of thechrominance-possessing macroblock position information using the encodedstream as an input, and writes the value of the obtained flag in theslice adaptive processing flag storage section 200-7.

The slice adaptive processing determination section 200-8 takes thevalue of the chroma-position-slice-adaptive read from the slice adaptiveprocessing flag storage section 200-7 as an input, performsdetermination processing to determine whether or not the value is 1, andoutputs a true or false value. If the output is a true value, then itwrites in the stream storage section 200-12 the encoded stream of thechrominance-possessing macroblock position information which follows onfrom the chroma-position-slice-adaptive flag in the encoded stream.

If, however, the output is not a true value, then the encoded stream isinput into the chrominance shared block set adaptive processing flagdetection section 200-9.

The chrominance shared block set adaptive processing flag detectionsection 200-9 detects the chroma-position-MBs-adaptive flag whichrelates to the setting of the chrominance-possessing macroblock positioninformation using the encoded stream as an input, and writes the valueof the obtained flag in the chrominance shared block set adaptiveprocessing flag storage section 200-10.

The chrominance shared block set adaptive processing determinationsection 200-11 takes the value of the chroma-position-MBs-adaptive readfrom the chrominance shared block set adaptive processing flag storagesection 200-10 as an input, performs determination processing todetermine whether or not the value is 1, and outputs a true or falsevalue. If the output is a true value, then it writes in the streamstorage section 200-12 the encoded stream of the chrominance-possessingmacroblock position information which follows on from thechroma-position-MBs-adaptive flag in the encoded stream.

The chrominance-possessing macroblock position information decodingsection 200-13 performs processing to decode the chrominance-possessingmacroblock position information using the encoded stream read from thestream storage section 200-12 as an input, and writes the decodingresult in the chrominance-possessing macroblock position informationstorage section 204.

The luminance signal decoding processing section 201 performs processingto decode luminance signals using the encoded stream as an input, andwrites the result in the luminance signal storage section 202.

The chrominance-possessing macroblock position information determinationsection 205 takes the chrominance-possessing macroblock positioninformation read from the chrominance-possessing macroblock positioninformation storage section 204 as an input, and determines whether ornot a macroblock targeted for encoding is in a chrominance-possessingmacroblock position. If it is determined that it is in this position,then the chrominance-possessing macroblock position informationdetermination section 205 sends a control signal such that theprocessing of the chrominance signal decoding processing section 206 isperformed.

The chrominance signal decoding processing section 206 performsprocessing to decode chrominance signals using the encoded stream as aninput, and writes the result in the chrominance signal storage section207.

The chrominance signal upsampling processing section 208 takesdownsampling ratios read from the downsampling ratios storage section203 and chrominance signals read from the chrominance signal storagesection 207 as an input, and calculates upsampling ratios based on thedownsampling ratios, and then performs upsampling processing for thesechrominance signals. It then writes the processed chrominance signals inthe chrominance signal storage section 209.

In this manner, the video decoding apparatus shown in FIG. 14 executesthe decoding processing shown in the flow charts in FIG. 11 and FIG. 12in accordance with the apparatus structure shown in this FIG. 14.

Next, as is described above in the seventh aspect, in the presentinvention, when a downsampling ratio is changed adaptively in units offrames, it is possible to generate an interpolated image of a referenceframe which realizes the required motion compensation.

FIG. 15 shows an embodiment of the reference frame interpolation imagegeneration apparatus of the present invention which realizes thisseventh aspect.

The reference frame interpolation image generation apparatus of thepresent invention which is constructed in this manner is used in thegeneration of a reference signal which is used in the motion estimationof a chrominance signal. This reference frame interpolation imagegeneration apparatus may be applied in a reference signal generationprocessing section 1000 provided in a video encoding apparatus whichemploys a structure such as that shown in FIG. 16, or may be applied ina reference signal generation processing section 2000 provided in avideo decoding apparatus which employs a structure such as that shown inFIG. 17.

Here, in FIGS. 15, 300 and 301 are an interpolation filter coefficientstorage section, 302 is a reference frame signal storage section, 303and 304 are downsampling ratio storage sections, 305 is a referenceframe interpolation accuracy calculation section, 306 is a referenceframe interpolation accuracy storage section, 307 is a reference frameinterpolation processing determination section, 308 is a reference frameinterpolation pixel value generation section, 309 is a reference frameinterpolation pixel value storage section, 310 is an output controlunit, and 311 is a reference frame interpolation processing statestorage section.

The interpolation filter coefficient storage section 300 storesinterpolation filter coefficients for reference frames in motioncompensation.

In the same way, the interpolation filter coefficient storage section301 stores interpolation filter coefficients for reference frames inmotion compensation. In the present embodiment, in order to illustratean example in which two types of filter coefficients are usedselectively, two filter coefficients are stored respectively inindividual storage sections.

For example, these filter coefficients are switched in accordance withthe interpolation position (i.e., a 1/2 pixel position, or a 1/4 pixelposition) in the motion compensation. Note that the present inventioncan be applied in the same way when two or more types of filtercoefficient are used.

The reference frame signal storage section 302 stores reference framesignals in the motion compensation.

The downsampling ratio storage section 303 stores downsampling ratiosfor reference frames in the motion compensation.

The downsampling ratio storage section 304 stores downsampling ratiosfor target frames (i.e., frames targeted for encoding and framestargeted for decoding) in the motion compensation.

The reference frame interpolation accuracy calculation section 305 usesas inputs the downsampling ratios for reference frames read from thedownsampling ratio storage section 303, and the downsampling ratios fortarget frames read from the downsampling ratio storage section 304,performs processing to calculate the interpolation accuracy required ina reference frame, and writes this in the reference frame interpolationaccuracy storage section 306. The specific calculation methodcorresponds to Formula (1).

The reference frame interpolation processing determination section 307uses as inputs interpolation accuracies of reference frames read fromthe reference frame interpolation accuracy storage section 306, andinformation (for example, information showing that interpolationprocessing is already completed for 1/2 pixel positions) showing thepositions of pixels which have already undergone interpolationprocessing which is read from the reference frame interpolationprocessing state storage section 311, and determines whether or not allinterpolated pixel values for a reference frame have been generated. Ifthe result of this determination is a true value, the output controlunit 310 is driven, and the value in the reference frame interpolationpixel value storage section 309 is output (namely, the final referenceframe interpolation image is output).

If, however, the determination result is a false value, the routinemoves to the processing of the reference frame interpolation pixel valuegeneration section 308. At this time, information showing the positionof the pixel targeted for interpolation is given to the reference frameinterpolation pixel value generation section 308.

The reference frame interpolation pixel value generation section 308takes information showing the position of a pixel targeted forinterpolation as an input, and, in accordance with this information,reads filter coefficients from the interpolation filter coefficientstorage section 300 or the interpolation filter coefficient storagesection 301 as inputs.

Next, interpolation processing is performed using as inputs the pixelvalues of the reference frame read from the reference frame signalstorage section 302, or the pixel values of the reference frame whichalso includes interpolated positions read from the reference frameinterpolation pixel value storage section 309, and the result of thisinterpolation processing is written in the reference frame interpolationpixel value storage section 309.

Note that the reference frame interpolation pixel value storage section309 at the start is initialized with a value of 0. Moreover, each timethe processing for each reference frame ends, in the same way, it isinitialized with a value of 0. In this manner, in this processing, aninitial value is not read from the reference frame interpolation pixelvalue storage section 309 if the initial value is stored therein.

Furthermore, information (for example, information showing thatinterpolation processing is already completed for 1/2 pixel positions)showing the position of pixels which have already completed theinterpolation processing is written in the reference frame interpolationprocessing state storage section 311.

In this manner, the reference frame interpolation image generationapparatus of the present invention which is shown in FIG. 15 performsprocessing such that when a structure in which downsampling ratios arechanged adaptively in units of frames is employed in a video encodingapparatus which employs a structure such as that shown in FIG. 16 or ina video decoding apparatus which employs a structure such as that shownin FIG. 17, an interpolated image of a reference frame which enables therequired motion compensation to be achieved is generated in accordancewith the interpolation accuracy of the reference frame which changesdepending on the structure thereof.

FIG. 18 shows an embodiment of a flowchart executed by a reference frameinterpolation image generation apparatus which employs the apparatusstructure shown in FIG. 15. A description of the processing executed bythis reference frame interpolation image generation apparatus will nowbe described in accordance with this flowchart.

Step S300: BR>F An interpolation filter coefficient for a referenceframe in motion compensation is read as an input, and written in theregister. Note that in the present invention, it is possible to use aplurality of types of filter coefficient.

Step S301: A downsampling ratio for the reference frame in motioncompensation is read as an input, and written in the register.

Step S302: A downsampling ratio for the target frame in motioncompensation is read as an input, and written in the register.

Step S303: A reference frame signal in motion compensation is read as aninput, and written in the register.

Step S304: Using the downsampling ratio for the reference frame and thedownsampling ratio for the target frame as inputs, processing isperformed in order to calculate the interpolation accuracy required inthe reference frame, and this is written in the register. The specificcalculation method used corresponds to Formula (1).

Step S305: Using the interpolation accuracy for the reference framecalculated in step S304 as an input, determination processing isperformed in order to determine whether or not the value is less thanone pixel, and either a true or false value is output. If the output isa true value, the routine moves to the processing of step S306, while ifthe output is a false value, processing is ended as it is not necessaryfor interpolation to be performed.

Step S306: Using the current interpolation accuracy of the referenceframe and the interpolation accuracy for the reference frame calculatedin step S304 as inputs, determination processing is performed in orderto determine whether or not the current interpolation accuracy of thereference frame is less precise than the interpolation accuracy of thereference frame calculated in step S304, and a true or false value isthen output. If the output is a true value, the routine moves to theprocessing of step S307, while if the output is a false value,processing is ended.

Step S307: Using the current interpolation accuracy of the referenceframe as an input, an interpolation filter coefficient of this accuracyis selected, and this filter coefficient is written in the register.

The specific filter coefficient (which is stored in the interpolationfilter coefficient storage section 300 or 301) is supplied from anexternally provided mechanism which determines filter coefficients.

Step S308: Using the reference frame signal, the current interpolationaccuracy of the reference frame, and the interpolation filtercoefficient of this accuracy which was selected in step S307 as inputs,interpolation processing is performed and an interpolated image of thereference frame is generated.

Step S309: Using the interpolation accuracy of the reference frame fromstep S308 as an input, this interpolation accuracy is made even moreprecise.

These levels of interpolation accuracy are as follows: if, for example,an interpolated image for a 1/8 pixel accuracy is desired, then bysequentially making the filter coefficient of the interpolation accuracymore and more precise, interpolated images are generated at positionscorresponding in sequence to 1/2, 1/4, and 1/8 pixel accuracies.

In this manner, as a result of the reference frame interpolation imagegeneration apparatus shown in FIG. 15 executing the reference frameinterpolation image generation processing of the flowchart shown in FIG.18, then when a structure is employed in which the downsampling ratio ischanged adaptively in units of frames, even if the interpolationaccuracy of the reference frame changes in accordance with a structurethereof, the processing makes it possible to generate an interpolatedimage of the reference frame which achieves the required motioncompensation.

INDUSTRIAL APPLICABILITY

The present invention is able to be applied when a video signal made upof two or more signal elements is targeted for encoding. When a videoimage is being encoded, it is possible to adaptively change thedownsampling ratio of a specific signal element, so that, as a result,it is possible to achieve an efficient reduction in the code amount inaccordance with the visual sensitivity.

The invention claimed is:
 1. A video encoding method in which a videosignal made up of two or more signal elements is targeted for encoding,comprising: a step of setting a downsampling ratio for a specific signalelement of a frame in accordance with a size of an amount of transitionwithin the frame by storing the downsampling ratio in a memory of avideo encoding apparatus; a step of generating a video signal which istargeted for encoding by using said video encoding apparatus todownsample the specific signal element inside the frame in accordancewith the set downsampling ratio stored in said memory; a step ofappending information showing whether or not each block of a specificsize includes a significant coefficient to each block for each signalelement quantized by said step of generating the video signal targetedfor encoding; and a step of setting position information showing theposition of a group of blocks which includes the specific signal elementwithin a set of groups of blocks which share the specific signalelement, for each set of groups of blocks.
 2. The video encoding methodaccording to claim 1, wherein the step of setting the downsampling ratiocomprises: using said video encoding apparatus to perform a step ofdividing the frame into partial areas in accordance with localizedcharacteristics within the frame; and using said video encodingapparatus to perform a step of setting downsampling ratios for specificsignal elements in the partial areas in accordance with thecharacteristics in the respective partial areas, and generating a videosignal targeted for encoding by downsampling the specific signalelements of the partial areas in accordance with the set downsamplingratios using said video encoding apparatus.
 3. A video decoding methodfor decoding encoded data of a video signal generated by the videoencoding method according to claim 1, comprising: using a video decodingapparatus to perform a step of determining as to whether or not a groupof blocks targeted for decoding is a group of blocks which includes thespecific signal element by decoding encoded data of the positioninformation showing at which position the group of blocks which includesthe specific signal element is located; and using the video decodingapparatus to perform a step of, for the group of blocks which has beendetermined in the determination step to be a group of blocks whichincludes the specific signal element, decoding a downsampled signalelement in accordance with a downsampling ratio set on an encoding sideby decoding encoded data of the specific signal element included in thegroup of blocks.
 4. A non-transitory video decoding program which isused to execute on a computer processing which is used to implement thevideo decoding method according to claim
 3. 5. A non-transitorycomputer-readable recording medium on which is recorded a video decodingprogram which is used to execute on a computer processing which is usedto implement the video decoding method according to claim
 3. 6. Anon-transitory video encoding program which is used to execute on acomputer processing which is used to implement the video encoding methodaccording to claim
 1. 7. A non-transitory computer-readable recordingmedium on which is recorded a video encoding program which is used toexecute on a computer processing which is used to implement the videoencoding method according to claim
 1. 8. A video encoding method inwhich a video signal made up of two or more signal elements is targetedfor encoding, comprising: a step of setting a downsampling ratio for aspecific signal element of a frame in accordance with characteristicsinside the frame by storing the downsampling ratio in a memory of avideo encoding apparatus; a step of generating a video signal which istargeted for encoding by using said video encoding apparatus todownsample the specific signal element inside the frame in accordancewith the set downsampling ratio stored in said memory; a step of, usingthe video encoding apparatus to test when motion compensation having adecimal fraction pixel accuracy is used for an inter-frame prediction inthe encoding of the video signal targeted for encoding, using the videoencoding apparatus to change the interpolation accuracy of a downsampledsignal element in a reference frame in accordance with a ratio betweenthe downsampling ratio of the frame targeted for encoding and thedownsampling ratio of the reference frame; and a step of using the videoencoding apparatus to generate an interpolated image of the referenceframe based on the changed interpolation accuracy.
 9. A video encodingmethod in which a video signal made up of two or more signal elements istargeted for encoding, comprising: a step of setting a downsamplingratio for a specific signal element of a frame in accordance withcharacteristics inside the frame by storing the downsampling ratio in amemory of a video encoding apparatus; and a step of generating a videosignal which is targeted for encoding by using said video encodingapparatus to downsample the specific signal element inside the frame inaccordance with the set downsampling ratio stored in said memory,wherein the step of setting the downsampling ratio further comprises:using the video encoding apparatus to divide the frame into partialareas in accordance with localized characteristics within the frame; andusing the video encoding apparatus to set downsampling ratios forspecific signal elements in the partial areas in accordance with thecharacteristics in the respective partial areas, and wherein the step ofgenerating the video signal generates the video signal targeted forencoding by downsampling the specific signal elements of the partialareas in accordance with the set downsampling ratios, and the videoencoding method further comprises: using the video encoding apparatus toperform a step of, when motion compensation having a decimal fractionpixel accuracy is used for an inter-frame prediction in the encoding ofthe video signal targeted for encoding, the interpolation accuracy of adownsampled signal element in a partial area within the reference frameis changed in accordance with a ratio between a downsampling ratio ofeach of the partial areas within the frame targeted for encoding and adownsampling ratio of a partial area within a reference frame which isreferred to when each of the partial areas undergoes motioncompensation; and using the video encoding apparatus to perform a stepof generating an interpolated image of the partial area within thereference frame based on the changed interpolation accuracy.
 10. A videodecoding method comprising: using a video decoding apparatus to performa step of, when motion compensation having a decimal fraction pixelaccuracy is used for an inter-frame prediction in decoding of encodeddata of a video signal targeted for encoding which is made up of two ormore signal elements and is generated by downsampling a specific signalelement inside a frame based on a downsampling ratio that is set inaccordance with characteristics inside the frame, changing theinterpolation accuracy of a downsampled signal element in a referenceframe in accordance with a ratio between a downsampling ratio of a frametargeted for decoding and a downsampling ratio of the reference frame;and using the video decoding apparatus to perform a step of generatingan interpolated image of the reference frame based on the changedinterpolation accuracy.
 11. A non-transitory video decoding programwhich is used to execute on a computer processing which is used toimplement the video decoding method according to claim
 10. 12. Anon-transitory computer-readable recording medium on which is recorded avideo decoding program which is used to execute on a computer processingwhich is used to implement the video decoding method according to claim10.
 13. A video decoding method comprising: using a video decodingapparatus to perform a step of, when motion compensation having adecimal fraction pixel accuracy is used for an inter-frame prediction indecoding of encoded data of a video signal targeted for encoding whichis made up of two or more signal elements and is generated bydownsampling specific signal elements of partial areas of a frame thatare divided in accordance with localized characteristics inside theframe based on downsampling ratios that are set in accordance withcharacteristics in the respective partial areas, changing theinterpolation accuracy of a downsampled signal element in a partial areawithin a reference frame in accordance with a ratio between adownsampling ratio of each of the partial areas within a frame targetedfor decoding and a downsampling ratio of a partial area within thereference frame which is referred to when each of the partial areasundergoes motion compensation; and using the video decoding apparatus toperform a step of generating an interpolated image of the partial areawithin the reference frame based on the changed interpolation accuracy.14. A non-transitory video decoding program which is used to execute ona computer processing which is used to implement the video decodingmethod according to claim
 13. 15. A non-transitory computer-readablerecording medium on which is recorded a video decoding program which isused to execute on a computer processing which is used to implement thevideo decoding method according to claim
 13. 16. A video encodingapparatus which targets for encoding a video signal made up of two ormore signal elements, comprising: a device which sets a downsamplingratio for a specific signal element of a frame in accordance with a sizeof an amount of transition within the frame; a device which generates avideo signal which is targeted for encoding by downsampling the specificsignal element inside the frame in accordance with the set downsamplingratio; and a device which, when information showing whether or not eachblock of a specific size includes a significant coefficient is appendedto each block for each quantized signal element obtained by encoding thevideo signal targeted for encoding, sets position information showingthe position of a group of blocks which includes the specific signalelement within a set of groups of blocks which share the specific signalelement, for each set of groups of blocks.
 17. The video encodingapparatus according to claim 16, wherein the device which sets thedownsampling ratio comprises: a device which divides the frame intopartial areas in accordance with localized characteristics within theframe; and a device which sets downsampling ratios for specific signalelements in the partial areas in accordance with the characteristics inthe respective partial areas, and the video signal targeted for encodingis generated by downsampling the specific signal elements of the partialareas in accordance with the set downsampling ratios.
 18. A videodecoding apparatus which decodes encoded data of a video signalgenerated by the video encoding apparatus according to claim 16,comprising: a device which determines as to whether or not a group ofblocks targeted for decoding is a group of blocks which includes thespecific signal element by decoding encoded data of the positioninformation showing at which position the group of blocks which includesthe specific signal element is located; and a device which, for thegroup of blocks which the determination device has determined to be agroup of blocks which includes the specific signal element, decodes adownsampled signal element in accordance with a downsampling ratio seton an encoding side by decoding encoded data of the specific signalelement included in the group of blocks.
 19. A video encoding apparatuswhich targets for encoding a video signal made up of two or more signalelements, comprising: a device which sets a downsampling ratio for aspecific signal element of a frame in accordance with thecharacteristics inside the frame; a device which generates a videosignal which is targeted for encoding by downsampling the specificsignal element inside the frame in accordance with the set downsamplingratio; a device which, when motion compensation having a decimalfraction pixel accuracy is used for an inter-frame prediction in theencoding of the video signal targeted for encoding, changes theinterpolation accuracy of a downsampled signal element in a referenceframe in accordance with a ratio between the downsampling ratio of theframe targeted for encoding and the downsampling ratio of the referenceframe; and a device which generates an interpolated image of thereference frame based on the changed interpolation accuracy.
 20. A videoencoding apparatus which targets for encoding a video signal made up oftwo or more signal elements, comprising: a device which sets adownsampling ratio for a specific signal element of a frame inaccordance with the characteristics inside the frame; and a device whichgenerates a video signal which is targeted for encoding by downsamplingthe specific signal element inside the frame in accordance with the setdownsampling ratio, wherein the device which sets the downsampling ratiocomprises: a device which divides the frame into partial areas inaccordance with localized characteristics within the frame; and a devicewhich sets downsampling ratios for specific signal elements in thepartial areas in accordance with the characteristics in the respectivepartial areas, and the video signal targeted for encoding is generatedby downsampling the specific signal elements of the partial areas inaccordance with the set downsampling ratios, and the video encodingapparatus further comprises: a device which, when motion compensationhaving a decimal fraction pixel accuracy is used for an inter-frameprediction in the encoding of the video signal targeted for encoding,changes the interpolation accuracy of a downsampled signal element in apartial area within the reference frame in accordance with a ratiobetween a downsampling ratio of each of the partial areas within theframe targeted for encoding and a downsampling ratio of a partial areawithin a reference frame which is referred to when each of the partialareas undergoes motion compensation; and a device which generates aninterpolated image of the partial area within the reference frame basedon the changed interpolation accuracy.
 21. A video decoding apparatuscomprising: a device which, when motion compensation having a decimalfraction pixel accuracy is used for an inter-frame prediction indecoding of encoded data of a video signal targeted for encoding whichis made up of two or more signal elements and is generated bydownsampling a specific signal element inside a frame based on adownsampling ratio that is set in accordance with characteristics insidethe frame, changes the interpolation accuracy of a downsampled signalelement in a reference frame in accordance with a ratio between adownsampling ratio of a frame targeted for decoding and a downsamplingratio of the reference frame; and a device which generates aninterpolated image of the reference frame based on the changedinterpolation accuracy.
 22. A video decoding apparatus comprising: adevice which, when motion compensation having a decimal fraction pixelaccuracy is used for an inter-frame prediction in decoding of encodeddata of a video signal targeted for encoding which is made up of two ormore signal elements and is generated by downsampling specific signalelements of partial areas of a frame that are divided in accordance withlocalized characteristics within the frame based on downsampling ratiosthat are set in accordance with characteristics in the respectivepartial areas, changes the interpolation accuracy of a downsampledsignal element in a partial area within a reference frame in accordancewith a ratio between a downsampling ratio of each of the partial areaswithin a frame targeted for decoding and a downsampling ratio of apartial area within the reference frame which is referred to when eachof the partial areas undergoes motion compensation; and a device whichgenerates an interpolated image of the partial area within the referenceframe based on the changed interpolation accuracy.